158 GENERAL SCIENCE 



of gravity. The pull of the earth gives weight to bodies, and causes 

 the familiar phenomenon of falling bodies. There is sufficient reason 

 for believing that every particle of matter, regardless of where it may 

 be, exerts a pull upon all other matter everywhere. Why it does this 

 is unknown. The value of the force of gravitation depends upon the 

 amount of matter in the bodies attracting each other, and upon the 

 distances separating them. 



FIG. 54. Alighting from a street car. When facing forward one is less 

 likely to be thrown if the car suddenly starts. 



A similar attraction exists between the minute particles known as 

 molecules of which matter is supposed to be made up. The distance 

 through which this molecular force is exerted, however, is too small for 

 any actual measurement. 



Exercises 



1. Distinguish between mass and density of bodies. 



2. What is meant by the properties of bodies? 



3. Name four forms of strain. Of these, which are involved in testing the 

 tenacity of a piece of wire? 



4. When mercury is spilled and breaks up into globules, why do they retain a 

 spherical form as they roll about? 



6. What is meant by the ductility of metals? Mention several uses of iron 

 when in the form of wires. 



